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Common TCP/IP Applications and Assigned Well-Known and Registered Port Numbers(Page 1 of 2)

The great popularity of the TCP/IP
protocol suite has led to the development of literally thousands of
different applications and protocols. Most of these use the client/server
model of operation that we discussed earlier in this section. Server
processes for a particular application are designed to use a particular
reserved
port number, with clients using an ephemeral
(temporary) port number to initiate a
connection to the server.

Management of Reserved Port Numbers

To ensure that everyone agrees on
which port numbers server applications for each application should use,
they are centrally managed by the Internet
Assigned Numbers Authority (IANA). Originally,
IANA kept the list of well-known and registered port numbers in a lengthy
text document, along with all the many other parameters for which IANA
was centrally responsible (such as IP Protocol field numbers,
Type
and Code field values for ICMP, and so
on). These were published on a periodic basis in Internet (RFC) standards
documents titled Assigned Numbers.

This system worked fine in the early
days of the Internet, but by the mid-1990s, these values were changing
so rapidly that using the RFC process was not feasible. It was too much
work to keep publishing them, and the RFC was practically out of date
the day after it was put out.

The last Assigned Numbers
standard was RFC 1700, published in October 1994. After that time, IANA
moved to a set of World Wide Web documents containing the parameters
they manage. This allowed IANA to keep the lists constantly up to date,
and for TCP/IP users to be able to get more current information. RFC
1700 was officially obsoleted in 2002.

The document mentioned above is the
definitive list of all well-known and registered TCP and UDP port assignments.
Each port number is assigned a short keyword, with a brief description
of the protocol that uses it. There are two problems with this document.
The first is that it is incredibly long: over 10,000 lines
of text. Most of the protocols mentioned in those thousands of lines
are for obscure applications that you have probably never heard of before
(I certainly have never heard of most of them!) This makes it hard to
easily see the port assignments for the protocols that are most commonly
used.

The other problem with this document
is that it shows the same port number as reserved for both TCP and UDP
for an application. As
I mentioned earlier, TCP and UDP port
numbers are actually independent, so one could in theory assign TCP
port 80 to one server application type and UDP port 80 to another. It
was believed that this would lead to confusion, so with very few exceptions,
the same port number is shown in the list for the same application for
both TCP and UDP. This makes sense, but showing this in the list has
a drawback: you can't tell which protocol the application actually uses,
and which has just been reserved for consistency.

Given all that, I've decided to include
a couple of summary tables here that show the well-known and registered
port numbers for the most common TCP/IP applications, and indicated
whether the protocol uses TCP, UDP or both.

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